Origin of Color (UV-VIS-NIR), Natural Unheated Colored Sapphires

Fig. 21 The GRS UV-VIS-NIR multi-channel spectrometer used for recording the absorption spectra of the gemstones. A special measuring chamber set-up (To right of picture) enables the collection of non-polarized, or polarized absorption spectra, between 290nm (UV) and 1024nm (NIR) on rough or faceted gemstones in all different crystallographic directions. Fig. 22a Absorption spectra of natural unheated Fancy Sapphires (origin Madagascar). The cause of color is a variable combination of chromium (Cr3+ single ions, band centered at 405-410nm and 554nm, subordinate sharp lines at 475, 477 and 693nm ), iron (Fe3+ single ions, line at 388nm, Fe3+-pairs, narrow lines at 377nm and 450nm and broad bands at 540nm and 700nm, See olive colored sample) and titanium concentrations (Fe2+-Ti4+ inter valence charge transfer, band centered at 565nm). See also Lit. 04 and 28. ED-XRF analyses, See Fig. 23. Fig. 22b The samples of four different colors of Natural Sapphires unenhanced by heat spectroscopically measured (Fig 22a). Fig. 23 Represenation of ED-XRF data on the dominant trace element concentrations (in oxide wt.-%) in natural colored sapphires unenhanced by heat (N) from Madagascar. Color groups: Y = yellow, greenish yellow, green; BO = brown, orange-pink; B = light blue, blue; LP = light purple and purple; P = purple violet

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